Changing Passwords

Microsoft explains how to 'change' a password, but does NOT explain what will follow when it IS 'changed'. So to avoid taking a leap into the dark, and taking a step whose consequences are unclear to me, I would like to know a little more about this passwords business.

a) Does setting a password prevent other network users, and specifically the administrator, from accessing my files and system via the network without knowing the password? Or is permission for network access from other computers set elsewhere, and if so, where?

(This is what I really want to do, to enhance network security and prevent my local computer being accessed by a hacker)

b) Do I have to key in this password every time I start the computer? At what stage in the start-up process does the request for a password appear, i.e., is it after all my programs have loaded, or when?

c) What is the connection between 'logging on' (a concept I have never understood) and 'passwords'?

d) What is the connection between 'administrator privileges' and 'passwords'? I.e., my question is, unless I also set passwords for the administrator (and I AM the administrator!) can someone who enters the system and logs in as 'administrator' overide my local password? If so, how do I set a password as 'administrator'?

e) What book would fellow group users recommend to the SOHO user such as myself to read, which would explain these and other basic points of network management and security?